The most commonly-used of the Player utilities, player is a TCP server that allows remote access to devices. It is normally executed on-board a robot, and is given a configuration file that maps the robot's hardware to Player devices, which are then accessible to client programs. The situation is essentially the same when running a simulation.

$ player pioneer.cfg
Part of the Player/Stage/Gazebo Project
[http://playerstage.sourceforge.net].
Copyright (C) 2000 - 2006 Brian Gerkey, Richard Vaughan, Andrew Howard,
Nate Koenig, and contributors. Released under the GNU General Public
License.
Player comes with ABSOLUTELY NO WARRANTY. This is free software, and you
are welcome to redistribute it under certain conditions; see COPYING
for details.
Listening on ports: 6665